The present invention relates to a method for the non-contaminating solidification of medium and low radioactivity aqueous waste liquids and/or waste liquid containing tritium compounds for storage, wherein the waste liquids are initially used to form pellets or granules which are thereafter embedded for storage purposes. When the waste contain easily leachable radionuclides, the granules or pellets are optionally clad or coated with a binder prior to being embedded within the same or different binder of the types set forth herein.
More than 20 years ago, it was proposed to solidify aqueous low radioactive (LAW) waste liquids by processing the radioactive wastes with hydraulic binders, e.g., cement, into transportable bodies. In order to achieve as uniform distribution of the radioactive substances in such a solid body as possible, and in an effort to accommodate as large of a quantity of the waste liquid in such a solid body as possible, absorbing substances, such as, for example, montmorillonite or heat treated vermiculite etc., were mixed with the cement. The hardened shaped bodies of said mixtures and aqueous LAW waste liquids, however, exhibited a relatively low resistance to leaching. The leaching rates for the harmful radionuclides cesium.sup.137 or strontium.sup.90 etc. were high and the aforementioned cement solidification processes thus proved to be unsatisfactory for aqueous LAW liquids and useless for medium radioactive category (MAW) liquids.
In an effort to overcome these disadvantages, attempts were then made to bind the radioactive waste waters or slurries in bitumen. According to this process, water was evaporated during the addition of the waste waters or slurries to the liquid bitumen, resulting in the solids and salts being enclosed in the bitumen. Due to its properties, the bitumen matrix could have been used not only for LAW liquids, but also for MAW liquids; these properties include larger volume reduction of the wastes, higher concentrations of the radioactive substances, better leaching resistance by 2 to 3 powers of ten as compared to the cement stone solid bodies.
However, it has been found that waste liquids containing salts, such as, e.g., sodium sulfate or sodium carbonate, resulted in the formation of bitumen products which have lost the otherwise good leaching properties of the bitumen waste salt products. Moreover, the bitumen waste products exhibit a relatively poor heat conductance. Processes similar to butumination, wherein organic polymers, for example, polyethylene, polyvinyl chloride, polystyrene, and polyurethane, are used as the matrix instead of bitumen, have also been proposed. These waste products, however, exhibit an undesirably low radiation resistance, particularly when MAW wastes are incorporated in the matrices.
In order to provide assurance against leaching, a non-corroding coating or lining, e.g., of cast resin, was recommended for the containers which were to receive the solid bodies of cement stone having radioactive substances incorporated therein (see German Pat. No. 1,082,993). This process, however, is complicated and expensive, and thus, not practical. Moreover, there is no assurance that leaching will not occur when the containers are deformed, for example, during the placement thereof into final storage.
Cement solidification processes have been essentially practiced in accordance with the following two techniques:
1. mixing within the barrel or drum; and PA1 2. mixing in a mixer, and filling metered quantities into the containers. PA1 1. discharge of the majority of the waste waters directly into the main sewage channel; PA1 2. partial evaporation into the atmosphere; PA1 3. pressing into storage rock underground; however this method requires the presence of suitable geological structures, possibly at the location of the reprocessing plant; and PA1 4. binding tritium containing waste waters with, for example, hydraulic binders, such as cements; however, this process leads to products which, (a) have relatively high tritium water vapor pressures, and (b) exhibit relatively rapid leaching of the tritiated water.
The disadvantage of the first process is the difficulty of obtaining high capacities and, in the second process, the mixer becomes easily clogged.
The following methods have been used or discussed for the treatment of liquids containing tritium compounds:
It is also significant to note that in large reprocessing systems (capacities of about 1500 tons or more per year), and in the case of highly spent fuel elements, the environment must not be charged with large quantities of tritium.
A need therefore exists for a useful solidification process for all, or at least almost all occurring aqueous waste liquids, i.e., for LAW and MAW waste liquids as well as for liquids containing tritium compounds. Such a process has not been available prior to this invention.